Yep, and if you need a new gas bottle they normally have a spot for old gas bottles at your local tip, just grab one and take it in.
haha, you have to pay a "donation" at our local tip, but you can't buy one from them because it's illegal for them to sell old bottles.
For the OP, I have seen a spring scale used to weigh bottles that I have got filled, but it would be a few years ago now. And the woman at one of the local servos refused to do a swap and go on one of their bottles that was out of date once, even though I explained that it was their bottle, and what the site says etc. The bottle was clean, no rust and clearly stickered, just 3 months out of date...Got too hard, as she was a stubborn old cow, so I took it up the road to another servo and no problem.